1. Field of the Invention
The invention relates in general to a substrate to which a laser technique is applied, and more particularly to a substrate applied to high frequency field.
2. Description of the Related Art
Micro-via on a substrate can be categorized by blind via, buried via and through via. Conventionally, the micro-via, of which the bore diameter is larger than 200 μm, is formed by mechanically drilling and others are formed by a laser technique.
The conventional laser technique is using a laser with an optical system to work on a target workpiece. A pulsed laser beam can be focused on the target workpiece by a mirror or a prism, and the density of the laser power ranges from 105 to 1015 Watts per square centimeter (W/cm2). Such high laser power density can drill on any materials.
Because the heat inputted by the laser beams is highly more than the heat reflected, transmitted or spread by the materials, the materials are heating very fast and evaporating to form a micro-via. Several continuing micro-via form a cutting line with a narrow width by relatively moving the laser beams and the workpiece so that the substrate is cut. Further, a proper assisting gas can be added during the cutting process to help chemical reaction or remove the molten ashes in the cutting line.
Referring to FIG. 1A, it is a lateral view of a conventional micro via structure to which a laser technique is applied. The integrated-layer substrate 100 includes a build-up layer 102 and a core layer 104. The micro-via structure is disposed on the build-up layer 102. The differential signal pair on the upper layer 110 is electrically connected to the differential signal pair on the lower layer 112 by a conductive layer 108.
Referring to FIG. 1B and FIG. 1C, FIG. 1B is a top view of a conventional micro via structure to which a laser technique is applied, and FIG. 1C is a three-dimensional view of a conventional micro via structure to which a laser technique is applied. A third dielectric layer 114 is formed in the micro-via structure 106 by a laser technique to separate the micro-via structure 106 into a first camber conductive layer 116 and a second cambered conductive layer 118. The differential signal pair 110 includes a first trace 1101 and a second trace 1102. The first trace 1101 is electrically connected to the right conductive column 116, and the second trace 1102 is electrically connected to the left conductive column 118.
Generally, the differential signal pair 110 is used to suppress the common mode noise as long as the distance between two traces of the differential signal pair 110 is constant and small. When the distance between two traces of the differential signal pair 110 is less, the coupling capacity is higher, and the common mode noise is easier to be suppressed.
However, as for the conventional micro-via structure 106, the first camber conductive layer 116 and the second camber conductive layer 118 are curve-shapes structures so that the distance there between changes and is larger than the distance between the first trace 1101 and the second trace 1102. As the result, the coupling capacity decreases due to the increasing of the distance, and the characteristic impedance of the differential signal pair 110 won't be kept in a constant. The reflection of high-speed signals and the noise interferences will be raised and may produce electromagnetic interference (EMI).